Many common metallurgical processes, such as iron and steel making, produce natural by-products and waste materials in the form of dust and sludge. These by-products often contain oxides of iron, zinc, lead and cadmium. In some cases, like the by-products from a blast furnace, this waste material may also contain carbon, oil and grease. Due to its high monetary value, it is often desirable to reclaim the iron in this material for reuse; however, the presence of zinc and lead oxides or, in some instances, the presence of oil and/or grease, in the waste material make attempts to purify the iron difficult and impractical.
One method of reclaiming iron from iron oxide wastes is by reducing iron oxides through the use of high temperatures. Traditionally iron has been extracted from iron oxides, by means of indirect reduction, whereby blast furnaces transform iron ore into iron, in a molten state. Such processes produce iron high in carbon content, often called pig iron.
In another method direct reduced iron (DRI) can be produced by the solid state reduction of charged iron oxides into metallic iron without the formation of liquid iron.
In both processes, a reducing agent, generally solid or gaseous in form, is used to heat the ore to the high temperatures required for the reduction reaction. Examples of reducing agents are coal and natural gas. The reduction of the iron ore takes place through the interaction with hydrogen or carbon monoxide. For example, where the reducing agent is coal, the coal burns in a combustion furnace with carbon dioxide in the air to form carbon monoxide (C+CO2=2CO Boudouard reaction). This carbon monoxide then reacts with the iron ores, in the presence of heat, to produce iron and carbon dioxide (Fe2O3+3CO=2Fe+3CO2).
DRI tends to be advantageous over other types of iron, such as steel making waste and pig iron, because it has a known composition, has fewer impurities and can be continuously charged in steel making furnaces. Also, DRI processes require temperatures of less than 1000 degrees Celsius, as compared to indirectly reduced iron, produced in blast furnaces, which can require liquid iron temperatures of 1450 degrees Celsius.
The reduction process, which requires mixing of the reducing agent and the iron oxide at high temperatures, can be carried out in hearth furnaces. Rotary hearth furnace technology has been described, for example, in U.S. Pat. No. 5,186,741 or 4,701,214.
With most current DRI production, the input material is prepared before it is charged into the rotary hearth furnace. That material preparation consists of mixing, pelletizing or briquetting and mostly drying. During about one revolution of the hearth, the pellets or briquettes react and are discharged from the rotating hearth furnace.
Due to the low thermal conductivity in the material used in these systems, the rotary hearth furnace temperature is considerably higher than the temperature required for the reaction in order to ensure that all the material in the pellet or briquette reaches the temperature required for the metallurgical reactions. In these systems, the temperature in the rotary hearth furnace casing is controlled by burners installed in the roof or the side walls of the rotary hearth furnace and is normally between 1200 and 1400 degrees Celsius.
Multiple hearth processes for DRI production have also been described, for example, in U.S. Pat. No. 6,395,057 which describes a cylindrical furnace lined with refractory material with several fixed hearths made of refractory material. It has a rotating shaft in the centre of the multiple hearth furnace and that shaft supports rotating arms with rabbles. These rabbles transport the material across the different hearths, and the material that is charged on the upper hearth drops from one hearth to the next until it exits the multiple hearth furnace from the bottom hearth. However, in this system, because the hearth remains stationary and the rabbles and arms rotate, there is a gap between the hearth and the walls and the rabbles which is required to avoid mechanical interference. This gap tends to fill up with material during the process, forming a dead layer of input material that cannot be processed of moved. Over time that material can also harden to create an interference with the rotating rabbles.
It tends to be desirable to provide apparatus, process, systems and methods to overcome problems existing in the production of DRI. For example, it tends to be desirable to provide apparatus, process, systems and/or methods for treating metal oxide fines to recover elemental iron from iron-bearing materials including, for example, iron-bearing ores, steel mill waste and other metallurgical process waste. Additionally, it may be advantageous to provide apparatus, process, systems and methods which provide improved heat transfer and reduces material preparation, for example, agglomeration of the material feed into pellets and briquettes. Further, it may be advantageous to provide apparatus, process, systems and methods whereby accretions of material feed can be avoided, and where accretions cannot be avoided, the hardened material between system parts can be removed easily. It may additionally be desirable to provide apparatus, systems and methods of producing DRI whereby energy efficiency may be increased by using heat from the chemical energy of process gases and volatile matter, to reach the temperatures required for the reduction process.